Method for manufacturing a high toughness steel without subjecting it to heat treatment

ABSTRACT

A HIGH TOUGHNESS STEEL ESPECIALLY ADAPTED TO BE USED AS A MATERIAL FOR CONSTRUCTIONS TO BE USED AT LOW TEMPERATURES IS MANUFACTURED BY THE STEPS OF MAKING AN INGOT BY ADDING TO A FUNDAMENTAL STEEL HAVING A COMPOSITION OF FROM 0.03% TO 0.13% CRBON, 0.10% TO 0.60% SILICON, FROM 0.20% TO 2.00% MANAGANESE, UP TO 0.035% ALUMINIUM AND THE BALANCE BEING IRON AND IMPURITIES, AT LEAST ONE ELEMENT SELECTED FROM THE GROUP CONSISTING BY WEIGHT OF UP TO 0.2% NIOBIUM, UP TO 0.2% VANADIUM, UP TO 0.15% TITANIUM, UP TO 0.20% ZIRCONIUM AND UP TO 0.30% TANTALUM; ROUGH HOT ROLLING OR FORGING THE INGOT TO FORM A BILLET OR SLAB; THE BILLET OR SLAB BEING COOLED DURING SAID ROLLING OR FORGING TO BELOW THE A1 TRANSFORMATION POINT; UNIFORMLY REHEATING THE BILLET OR SLAB FROM BELOW THE AR1 TRANSFORMATION POINT AT ITS FINAL ROLLING OR FORGING STAGE TO A TEMPERATURE WITHIN 150*C. ABOVE THE AC3 TRANSFORMATION POINT; WORKING THE BILLET OR SLAB TO CAUSE A MORE THAN 10% THICKNESS REDUCTION, SAID WORKING BEING COMPLETED AT A TEMPERATURE IN THE VINCINITY OF THE AR3 TRANSFORMATION POINT; AND AIR COOLING OR FURNACE COOLING THE SLAB OR BILLET TO ROOM TEMPERATURE.

Jim 21, 1912 KATAO MIYANO ETAL 3,673,007

2mm V-NOTCH CHARPY VALUE kg-m SUBJECTING IT TO HEAT TREATMENT Filed Nov. 14, 1969 FIG. I

I I I I I o -140 -120 -1oo-ao -e'o -4'o -20 6 +2'o +40 +60 +o TEST TEMPERATURE c United States Patent O US. Cl. 148-12 1 Claim ABSTRACT OF THE DISCLOSURE A high toughness steel especially adapted to be used as a material for constructions to be used at low temperatures is manufactured by the steps of making an ingot by adding to a fundamental steel having a composition of from 0.03% to 0.13% carbon, 0.10% to 0.60% silicon, from 0.20% to 2.00% manganese, up to 0.035%aluminium and the balance being iron and impurities, at least one element selected from the group consisting by weight of up to 0.2% niobium, up to 0.2% vanadium, up to 0.15% titanium, up to 0.20% zirconium and up to 0.30% tantalum; rough hot rolling or forging the ingot to form a billet or slab; the billet or slab being cooled during said rolling or forging to below the A transformation point; uniformly reheating the billet or slab from below the Ar transformation point at its final rolling or forging stage to a temperature within 150 C. above the Ac transformation point; Working the billet or slab to cause a more than thickness reduction, said working being completed at a temperature in the vicinity of the Ar;, transformation point; and air cooling or furnace cooling the slab or billet to room temperature.

BACKGROUND OF THE INVENTION This invention relates to a method for manufacturing asteel, and more particularly to a method for manufacturing a steel suitable for use at a low temperature and having high notch toughness and excellent ductility at a low temperature as well as good weldability in the state of as rolled or tempering subsequent to its Working.

Recently, in order to meet the growing demand for steels for use at low temperatures in the fields of liquid gas storage plants, chemical plants etc., various efforts have been made to economically and plentifully supply steels having necessary low temperature characteristics such as high notch toughness and excellent ductility, etc.

A high-nickel alloy steel for low temperature services or a high AlN-bearing steel for low temperature service is hitherto commonly widely used in such fields.

However, the high-nickel alloy steel is costly since it includes a great quantity of nickel, while the high AIN- bearing steel is also inevitably expensive because, in order to give it necessary characteristics, it needs to be subjected to heat treatment subseqeunt to its final working.

SUMMARY OF THE INVENTION It is a main object of the present invention to provide a method for manufacturing a steel having necessary low temperature characteristics such as high notch toughness and excellent ductility, etc.

It is a further object of the present invention to provide a method for manufacturing a steel having necessary low temperature characteristics by the combination of making an ingot comprising a separate or compound addition of steel-characteristics-improving elements such as V, Nb, Zr, Ti, Ta, etc. to a fundamental low carbon steel and subjecting the ingot to a reheat working process.

It is another object of the present invention to provide a method for manufacturing an inexpensive steel for use at a low temperature which neither contains Ni, nor needs heat treatment subsequent to its working.

According to the present invention, a method for manufacturing a steel having necessary low temperature characteristics is characterized by making a low carbon content low alloy steel ingot by adding to a fundamental steel comprising by weight of 0.030.13% C, 0.10-0.60% Si, 0.20-2.00% Mn, up to 0.035% A1 and balance Fe and impurities, at least one element selected from the group consisting of, by weight, up to 0.20% Nb, up to 0.20% V, up to 0.15% Ti, up to 0.20% Zr and up to 0.30% Ta. The ingot is then roughly hot rolled or forged into a slab or billet to cool to a temperature below the Ar transformation point of the steel. The slab or billet is then uniformly reheated to a temperature of less than C. above the Ac transformation point from the temperature below the Ar transformation point at the final stage of rolling or forging. The slab or billet is then worked to cause a more than 10% thickness reduction, the working being completed at a temperature in the vicinity of the Ar transformation point. The slab or billet is then air or fur- "nace cooled, whereby a fine granulating efiect due to precipitation of carbides as well as nitrides and a refining effect of crystal grains due to working and recrystallization takes place.

BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects, features and advantages of the invention will be more readily understood from the detailed description in conjunction with the following drawings in which:

FIG. 1 is a diagram showing the relation between 2 mm. V-notch Charpy absorbing energy values and testing temperatures of a steel manufactured according to the present invention and steels conventionally manufactured; and

FIGS. 2A and B are the microstructures (X400) of a steel manufactured by the present invention and a steel conventionally manufactured respectively.

DETAILED DESCRIPTION OF THE INVENTION A method for manufacturing a high toughness steel without subjecting it to heat treatment according to the present invention comprises the steps of making an ingot by adding to a fundamental low carbon steel having a composition by weight of from 0.03% to 0.13% C, from 0.10% to 0.60% Si, from 0.20% to 2.00% Mn, up to 0.035% A1 and the balance being Fe and impurities, at least one element selected from the group consising by weight of up to 0.20% Nb, up to 0.20% V, up to 0.15% Ti, up to 0.20% Zr and up to' 0.30%Ta; rolling or forging said ingot so that a slab or a billet is obtained and is cooled to below the A transformation point, uniformly reheating said slab or billet to a temperature within 150 C. above the Ac transformation point of said carbon steel, subjecting the slab or billet to a working such as rolling or forging to cause a more than 10% thickness reduction, the working being completed at a temperature in the vicinity of the Ar transformation point, and letting said slab or billet cool in the air or cool slowly.

In the following description, the reasons for limiting the amounts of the various components of the composition of the steel are explained.

Although carbon has effects to increase the strength of ferrite-pearlite steels and to slightly refine their crystal grains, its excess addition increases the quantity of pearlite to damage low temperature notch toughness and severely injure weldability. So the range of the amount of carbon is selected to be from 0.03% to 0.13% by weight.

Silicon and manganese are necessary in order to obtain good killed steels and eifective in increasing their strength. However, when their contents are in excess, the ductility characteristic of the fundamental steel deteriorates and its weldability is also damaged. That is why the contents of silicon and manganese are in the ranges of 0.10%- 0.60% and 0.20%-2.00% by weight, respectively.

- Aluminium effectively acts a deoxidizing element on steels, but also it worsens its hot workability and is the main cause of defects of steels due to non-metallic inclusion resulting from residual oxidates in steels. So its content is limited to the necessary minimum amount for obtaining good killed steels.

Niobium, vanadium, titanium, zirconium and tantalum have a deoxidizing effect and become very strong fixing agents of carbon and nitrogen contained in steels so that they have the effect of refining austenite crystal grains of steels. But they refine also ferrite crystal grains transformed during subsequent cooling of the steels. Further, carbides and nitrides of these elements contained in a partial solid solution state at an austenitizing temperature reprecipitate during relatively slow cooling to suppress crystalgrain growth so that they have a refining effect on the crystal grains. However, since their excess addition increase the amounts of the elements existing in forms other than carbides and nitrides, and adversely affect weldability as well as toughness and ductility, and further since they are an expensive element, their optimum ranges are determined thus also taking economy into consideration.

As beforementioned, in the method according to the present invention, the low carbon fine grain steel having chemical components as defined above is reheated to a temperature within 150 C. above the Ac transformation point, subjected to a working of more than represented by the thickness decreasing ratio in the course of its air cooling, and finished the working at a temperature near Ar, transformation point, so that the crystal grain refining effect is taken place.

The reasons why the steel heating temperature, the amount of working and the working finished temperature mental steel as solid solutions, so that the refining effect of austenite crystal grains is also missed. Further, the

amount of compounds are increased during the subsequent cooling resulting in damage of ductility and toughnessof the steels due to their partially remaining in the fundamental steel, because they have not been perfectly precipitated into the steels.'So the reheating temperature is selected to be above within 1150? C. above the Ac transformation point.

The deformed crystals of the steelsgenerated by subjecting .them to a working during cooling from the reheating temperature subsequently recrystalize so that crystal grain growth is carried out. However, too light working causes remarkable crystal grain growth after recrystallization, and too low working finishing temperature increases residual working strain and assists the heterogeneous properties of'steels. Thus, in the present invention, theamount of working to be given the steel is determined to cause a more than 10% thickness reduction, and the working is finished at a temperature near the Ar transformation point, whereby a fine grain steel providing sufficient ductility as well as toughness can be obtained at a state of as worked without being subjected to heat treatment.

Incidentally, the method according to the present invention can be equally applicable to any form of steels such as plates, tubes, profiles, bars etc.

Table 1 shows the results of analyses of varioussteels.

In Table l, the steels B-G are the representatives of those having chemical compositions according to the present invention, while the steel A is a hitherto known low, carbon content Si-Mn steel adapted as a reference steel..According to the present method, these steels were cooled to room temperature, thereafter uniformly heated again to the temperature of 920 C., and rolled again to have the thickness of 15 mm. in the course of its cooling in the air, the working being finished at the temperature of 760 C. or in the vicinity of the Ar; transformation. (In the present examples, in heating and cooling rate of 3 C./rnin., Ac and Ar;; transformation points are 842- 870 C. and 750-780" C. respectively.)

TABLE 1 Chemical composition: weight percent (check analysis; plate thickness 15 mm.)

0 Si Mn P a 2A! Nb v Ti Zr Ta TABLE 2 Tensile property Impact property Tensile Elonga- Reduction Ferrite strengt tlon, of area, VIE-70 C., vTrS, vTr 30, grain kgJmm. percent percent kg. m. 0. 0. size No.

47.0 44.2 79.5 0.5 -ss 7.7 52.1 43.1 01.0 33.7 -105 -1s0 9.2 53.2 39.0 70.0 21.0 -125 -153 11.0 51.4 41.3 78.0 32.0 -12s -105 10.7 51.9 43.8 70.2 20.5 --100 -120 0.2 52.4 42.0 75.7 17.4 -s0 -115 9.0 50. 2 41. 5 74. a 24.0 -90 -124 0. s

are determined to such values are explained as follows. The mechanical properties of the steels according to In the steels having chemical components in the ranges stated above, the austenite crystal grains are fined than those of usual low carbon steels. Such an effect resides mainly in that the carbides and nitrides of niobium, vanadium, titanium, zirconium, tantalum, etc. exist still as: stable compounds at a high temperature so that they suppress growth of austenite crystal grains. However, an excess heating causes these compounds to cohere and be coarsened and to promote their sloving into the fundathe present invention at a state of as Table 2.

The mechanical properties of the same steels which were worked according to the conventional process, that is, the steels which were not rolled into plates having the thickness of 15 mm. from the rolled'ingots (they were obtained by roughly hot rolling the rolled steels having the thickness of mm. from the temperature of 1,250? C.) are shownin Table 3. i

worked are shown in The mechanical properties shown in Tables 2 and 3 were all obtained at a state of as rolled and, as will be clearly seen from them, even in the hitherto known low carbon Si-Mn steel (Steel A), the improved property in toughness can be recognized when it is manufactured according to the present invention. In the steels B-G containing the carbide-nitride producing elements such as Nb, V, Ti, Zr, Ta, etc. the improvement in toughness is so remarkable that they have sufficient notch toughness to allow their use as steels for use at a extremely low temperature in the state of as rolled.

TABLE 3 Tensile property Impact property Tensile Elonga- Reduction Ferrite strength, tion, of area, VE-70" 0., vTr S, v'lr 30, grain kgJmm. percent percent kg. m. size No.

The curve A in FIG. 1 is a graph showing the relation 25 of up to 0.20% niobium, up to 0.20% vanadium, up to between 2 mm. V-notch Charpy absorbing energy values and test temperatures of the steel D, while curves B and C are those in a state of being normalized and as rolled respectively of the steel having the identical chemical components but manufactured by the conventional manufacturing process. From comparing these curves, it becomes apparent that the steel manufactured according to the present invention provides remarkable notch toughness.

FIGS. 2A and B show respectively the microstructures of the steel manufactured by the method according to the present invention and the steel having the identical chemical components but manufactured by a conventional method, whereby the magnitude is 400 magnifications and the former was in the state of as rolled, while the latter was normalized. As viewed in FIG. 2, it will be understood that the microstructure of the steel A is a very fine ferrite-pearlite.

Thus, according to the present invention a steel having excellent notch toughness can be obtained by a combination of unique constitutional elements and working process.

The steels manufactured by the present invention are most suitable as constructional materials for various con- 0.15% titanium, up to 0.20% zirconium and up to 0.30% tantalum; rough hot rolling or forging said ingot to obtain a slab or billet and air cooling to lower the temperature thereof below the Ar transformation point; uniformly reheating said slab or billet to a temperature within C. about the Ac transformation point of said carbon steel; subjecting said slab or billet to a working such as rolling or forging to cause a more than 10% thickness reduction thereof, said working being completed at a temperature in the vicinity of the Ar transformation point; and air cooling or furnace cooling said slab or a billet to room temperature.

References Cited UNITED STATES PATENTS 2,108,588 2/ 1938 Lawrence 14812 3,432,368 3/1969 Nakamura 148-12 L. DEWAYNE RUTLEDGE, Primary Examiner W. W. STALLARD, Assistant Examiner US. Cl. X.R. 148-123 

